The present invention relates to a screw tightening control system and a screw tightening control method that control plural screw tightening apparatuses while synchronizing them.
In a case in which plural screws are tightened by using a screw tightening apparatus at a screw tightening process in a production line for products, there is a method in which, in order to prevent a workpiece into which the screws are tightened from inclining or rotating, a step is repeated in which some of the plural screws are tightened until a tightening torque reaches a predetermined level, and the other screws are tightened until tightening torque reaches the predetermined level (for example, refer to Japanese Patent Laid-Open No. 9-141566).
However, in this method, it is necessary to perform shifting and positioning of the screw tightening apparatus every time a screw to be tightened is changed, which takes a long time until all the screws are tightened with a final torque.
Therefore, in Japanese Patent No. 3569619, there is disclosed a method in which plural screw tightening apparatuses corresponding to the number of screws are prepared, and output torques for the plural screw tightening apparatuses are increased step by step while synchronizing them.
However, there in a variation in its operation characteristic due to an individual difference in a screw tightening apparatus. Therefore, even if an instruction for torque increase from a current same tightening torque to a same target torque is issued to plural screw tightening apparatuses, all the screw tightening apparatuses do not necessarily reach the target torque, which results in time differences thereamong.
Therefore, in a screw tightening control method disclosed in Japanese Patent No. 3569619, a signal indicating a synchronization waiting state is output from a screw tightening apparatus which has reached the target torque, and a synchronization determination is performed when the signal is output from all the screw tightening apparatuses, and the routine proceeds to a step for torque increase up to the next target torque.
However, in the screw tightening control method disclosed in Japanese Patent No. 3569619, the torque increase for all the screw tightening apparatuses is simultaneously started.
In this case, when a rotation of one screw between adjacent screws is progressed prior to the other screw, the workpiece into which the screws are being tightened starts inclining. Then, once the workpiece starts inclining, the inclination is made increasingly greater due to the contact between the screws and screw holes in the workpiece even the screws are tightened with the same torque.
Further, when the torque increase for all the screw tightening apparatuses is simultaneously started, because a turning force applied to the workpiece into which the screws are tightened is increased, a retaining force capable of overcoming the turning force is required.
There is a method in which the same number of right-handed screws and left-handed screws are used so as to balance out such a turning force. However, in this case, not only special left-handed screws are required, but also two systems for the right-handed screws and the left-handed screws are required as a system for supplying screws to a production line.